Plastic materials have been used for the manufacture of ophthalmic lenses for many years. Plastics offer advantages to the patient over glass, most noticeably in their lower densities, allowing for a lighter lens, and greater impact strength. Conversely, plastic lenses can exhibit disadvantages: they tend to scratch more easily, have higher levels of chromatic aberration (lower ABBE values), and may distort at higher temperature processing conditions, due to lower glass transition (Tg) temperatures, resulting in "warped" lenses, or lenses with high levels of optical distortion. Additionally, plastic lenses usually have lower index of refraction values when compared to glass, which tends to require lenses of increased thickness and reduced cosmetic appeal. The standard "CR-39" type lens, made from diethylene glycol bis(allyl carbonate), has an index of refraction of 1.498.
Advances in technology have allowed improvements in plastic lens performance. Coatings have been developed which impart improved scratch resistance. Some plastics have relatively high ABBE values which are adequate for minimizing the effects of chromatic aberration. Improvements in machining and optical lens processing equipment and processes have permitted the use of materials having lower glass transition temperatures. Plastics with higher indices of refraction, and physical lens design improvements, have helped improve the cosmetic appeal of plastic lenses.
While these improvements have helped the plastic lens gain acceptance in the marketplace, to a point where plastic lenses constitute a majority of lens eyewear in the U.S., increasing expectations for performance have dictated that good ophthalmic lenses have the following key properties:
Clarity and Color
Lenses which are "hazy" are obviously unacceptable to the patient. Color is also of concern both from the standpoint of having a "water-white" quality for best cosmetic appeal when the lens is first purchased, as well as maintenance of that color over time, e.g., being resistant to the effects of sunlight, which over time can cause the lens to turn yellow.
Optical Distortion
Lack of optical distortion requires a material having a relatively high ABBE number such that chromatic aberration is minimized; and, more importantly, a material having excellent uniformity in composition such that the occurrence of visible "waves" is minimized.
Rapidly Tintability
Many ophthalmic lenses are manufactured in semi-finished form and shipped to optical laboratories where the prescription is "ground in". With ever increasing emphasis on short optical lab turn-around times, e.g., 1 hour service, the ability of the lens material to rapidly accept fashion tints is important. In the case of semi-finished lens products, the front surface of the lens may have a scratch resistant coating which does not accept tint. Thus, the only route for tinting to occur may be the parent lens material on the back surface. This material must be tintable.
High Index of Refraction and Low Density
The higher the index of refraction, the thinner the finished lens will be for a given design. This higher index, especially when combined with a relatively low density, will allow for the manufacture of "thinner and lighter" lens products.
Over the past several years, plastic ophthalmic lenses have been fabricated from a variety of materials including polycarbonate and polymethylmethacrylate, as well as polymerized allylic compounds, epoxies, and urethanes. The most common plastic ophthalmic lens, however, is made from diethylene glycol bis(allyl carbonate) often referred to as "CR-39" (a specific product manufactured by PPG Industries). As previously mentioned, this material has a refractive index of 1.498. It is easily processed in optical laboratories, is able to be manufactured with low optical distortion and is readily tinted by various commercially available tinting dyes.
The use of polyester materials to produce ophthalmic lenses has been previously disclosed in various U.S. patents. Examples of such disclosures are U.S. Pat. Nos. 3,391,224, 3,513,224 and published PCT application WO 93/21010. U.S. Pat. No. 3,391,224 discloses a composition in which a polyester is combined with from 5 to 20 weight percent methyl methacrylate and less than 5 weight percent styrene to produce a thermosetting product which can be used to produce an ophthalmic lens. U.S. Pat. No. 3,513,224 discloses a composition in which 70 to 75 weight percent of a specific unsaturated polyester formed from the reaction of fumaric acid with triethylene glycol and 2,2-dimethyl-1,3-propanediol (otherwise known as neopentyl glycol) is combined with about 12 to 18 weight percent styrene and 8 to 12 weight percent ethylene glycol dimethacrylate. The styrene raises the index of refraction to approximately 1.52, and the ethylene glycol dimethacrylate reduces brittleness of the polymer.
A number of commercially available unsaturated polyester resins have been developed which are clear when cast and have a refractive index of approximately 1.56 (the high index being primarily attributable to the use of styrene as a cross-linking diluent monomer at a level of approximately 30 to 45 weight percent). For clarity, the terms "polyester resin" and "unsaturated polyester resin", shall mean the polyester resin only, without considering any amount of diluent monomer, such as styrene, that typically is included with the polyester resin when purchased. Likewise, when considering the percent by weight of polyester resin in the compositions disclosed throughout this patent, the percentages are calculated as polyester only, without considering any diluent monomer. The densities of the various polyester systems are also quite low (on the order of 1.25 grams/cc). These properties are superior to CR-39 (index of 1.498 and density of 1.32 grams/cc) with regard to the potential to make "thinner and lighter" lenses.
Polyester resins can be manufactured using different compositions to achieve a wide variety of physical properties (hard, soft, rigid, flexible, and the like). Typical commercial polyesters include those made from a variety of glycols and acids. Common glycols used in alkyd polyester synthesis include: ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, neopentyl glycol, and the like. Common acids used include: phthalic anhydride, isophthalic acid, adipic acid, and the like, used in conjunction with maleic anhydride and/or fumaric acid to provide unsaturation for cross-linking, usually with styrene or other diluent monomers.
Resins made using phthalic anhydride are commonly called "ortho resins"; those made with isophthalic acid are commonly referred to as "iso resins". With respect to properties desirable for making ophthalmic lenses, typical iso resins which have good scratch resistance are generally quite slow to tint. Typical ortho resins, on the other hand, are generally more scratch-prone, but tint more readily. Unsaturated polyester resins can demonstrate a propensity to polymerize somewhat non-uniformly causing internal optical distortion or visible "waves".
Polymerization of the polyester resin can be carried out in a number of ways. Quite common is the use of a system promoted with a material such as cobalt octoate or cobalt naphthenate. When used with methyl ethyl ketone peroxide, the system can be cured near room temperature. Other free radical polymerization techniques also can be used, including thermal curing, for example, using peroxides or diazo compounds, as well as photoinitiated curing using compounds selected from the following classes of photoinitiators: benzoin ethers, benzophenones, thioxanthones, ketals, benzyl dialkyl ketals, .alpha.-hydroxy ketones, substituted morpholino ketones, acetophenones, phosphine oxides, xanthones, and visible light photoinitiators, including: (a) fluorone dye/onium salt amine systems, (b) dye/borate systems, and (c) borate photoinitiators. Previously, photoinitiated curing of polyesters has been performed primarily for uses other than ophthalmic lenses so that optical distortion was not a concern. Osborn U.S. Pat. No. 3,650,669 discloses photopolymerization of polyesters, for purposes other than ophthalmic lenses, using high intensity light radiation.
Among the problems associated with casting and curing ophthalmic lenses by means of free-radical polymerization of various compounds is the maintenance of the above-described key properties of the lens. These key properties, particularly the "water-white" color for cosmetic appeal and stability of that color over time (e.g., being resistant to the effects of sun exposure), lack of optical distortion (i.e., the occurrence of visible "waves" is minimized), and tintability, are essential for commercial acceptance. Additionally, consumer advantages can be obtained if the lens can be made absorbent to ultraviolet light. Materials with a high index of refraction and low density will allow for the manufacture of "thinner and lighter" lens products.
Formulations based on unsaturated polyester resins can be utilized to produce high index, low density ophthalmic lenses. Low levels of optical distortion and good tint and color characteristics can be attained when the resin is modified with certain co-monomers and an exotherm depressant to improve tint speed and reduce optical distortion, as disclosed in my parent patent application.
The control of exotherm by means of gradual dissipation of the heat of polymerization has been regarded as being essential to obtaining good optical uniformity. The addition of an exotherm depressant to the polyester formulation, as disclosed in my parent application, helps to slow the rate of reaction, allowing for gradual dissipation of heat and the avoidance of "hot spots" and convection currents which result in optical distortion as the formulation gels and solidifies during cure. In other less reactive lens casting formulations, such as those incorporating diethylene glycol bis (allyl carbonate), or "CR-39", the slowing of the reaction is commonly achieved by adjustment of the cure cycle for a given initiator level. The beginning part of the cure cycle tends to be at a relatively low temperature and is nearly constant or slightly increasing in temperature during the initial stages of polymerization and gelation. Gelation usually occurs in about 0.5 to 1 hour, with the initial stage of polymerization being quite lengthy, typically, 1 to 6 hours. The complete cure cycle is usually 7 to 20 hours.
In experimentation leading to the present invention, initial attempts to replace a thermal polymerization initiator with a photoinitiator resulted in a lens that was yellow in color and/or had a high degree of optical distortion. Efforts to rotate the cast mold assemblies to provide the polyester with homogeneous light exposure over its entire surface did not solve the color or distortion problems. Attempts at using low intensity bulbs of short and long wavelengths also did not solve the optical distortion problem. Attempts to increase the exotherm depressant, found to be necessary for thermal curing of the composition to reduce optical distortion, e.g., .alpha.-methyl styrene, surprisingly worsened the optical distortion problem. As a result of these surprising results, eventually, and contrary to the teachings of the prior art, attempts were made to photocure the polyester as fast as possible, by eliminating the ce-methyl styrene exotherm depressant and using a relatively high intensity light source, e.g., 300 microwatts/cm.sup.2, 500 microwatts/cm.sup.2 and higher. Quite surprisingly, it was found that when the unsaturated polyester resins of the present invention were photocured very quickly, e.g., gelled in about 7 minutes or less, preferably in about 5 minutes or less, as defined by a gel test described in detail to follow, e.g., by using a relatively high intensity bulb, the completed lens actually had less striation or optical distortion, and the lens was harder and more scratch resistant, with less stress in the lens.
The compositions described here are curable in less than about 3 hours, preferably less than about 2 hours, most preferably less than about 1 hour.